ADAPTIVE FEEDBACK CANCELLATION BASED ON INSERTED AND/OR INTRINSIC CHARACTERISTICS AND MATCHED RETRIEVAL

• US 20110150257A1
• Filed: 04/02/2009
• Published: 06/23/2011
• Est. Priority Date: 04/02/2009
• Status: Active Grant

• Current Assignee: Oticon A/S (William Demant Fonden)
• Original Assignee: Oticon A/S (William Demant Fonden)
• Inventors: Elmedyb, Thomas Bo, Jensen, Jesper
• Granted Patent: US 8,442,251 B2
• Time in Patent Office: Days
• US Class Current: 381/318
• CPC Class Codes: H04R 25/453 electronically
• CPC Class Codes: H04R 3/02 for preventing acoustic reaction , i.e. acoustic oscillatory feedback specially adapted for hearing aids H04R25/453

Abstract

The invention relates to an audio processing system for processing an input sound to an output sound. The invention further relates to a method of estimating a feedback transfer function in an audio processing system. The object of the present invention is to provide an alternative scheme for minimizing feedback in audio processing systems. The problem is solved in that the audio processing system comprises an input transducer for converting an input sound to an electric input signal and defining an input side, an output transducer for converting a processed electric output signal to an output sound and defining an output side, a forward path being defined between the input transducer and the output transducer, and comprising a signal processing unit adapted for processing an SPU-input signal originating from the electric input signal and to provide a processed SPU-output signal, and an electric feedback loop from the output side to the input side comprising a feedback path estimation unit for estimating an acoustic feedback transfer function from the output transducer to the input transducer, and a enhancement unit for estimating noise-like signal components in the electric signal of the forward path and providing a noise signal estimate output, wherein the feedback path estimation unit is adapted to use the noise signal estimate output in the estimation of the acoustic feedback transfer function. This has the advantage of providing an adaptive feedback cancellation system which is robust in situations with a high degree of correlation between the output signal and the input signal of an audio processing system, e.g. a listening device. The invention may e.g. be used in public address systems, entertainment systems, hearing aids, head sets, mobile phones, wearable/portable communication devices, etc.

Claims

1. An audio processing system for processing an input sound to an output sound, the audio processing system comprising

an input transducer for converting an input sound to an electric input signal and defining an input side,

an output transducer for converting a processed electric output signal to an output sound and defining an output side,

a forward path being defined between the input transducer and the output transducer, and comprising a signal processing unit adapted for processing an SPU-input signal originating from the electric input signal and to provide a processed SPU-output signal, and

an electric feedback loop from the output side to the input side comprising

a feedback path estimation unit for estimating an acoustic feedback transfer function from the output transducer to the input transducer, and

an enhancement unit for extracting characteristics of an electric signal of the forward path and providing an estimated characteristics output;

wherein the feedback path estimation unit is adapted to use the estimated characteristics output in the estimation of the acoustic feedback transfer function.

2. An audio processing system according to claim 1 wherein said feedback path estimation unit comprises an adaptive filter comprising a variable filter part and an algorithm part for updating filter coefficients of the variable filter part, the algorithm part being adapted to base the update at least partly on said estimated characteristics output from the enhancement unit.

3. An audio processing system according to claim 1 wherein the characteristics of the electric signal of the forward path are selected from the group comprising a modulation index, periodicity, correlation time, noise-like parts and combinations thereof.

4. An audio processing system according to claim 1, wherein the enhancement unit is adapted for retrieving intrinsic noise-like signal components in the electric signal of the forward path.

5. An audio processing system according to claim 4, wherein the correlation time N₁ of the noise signal estimate output from the enhancement unit obeys N₁≦

dG, where dG is the delay of the forward path.

6. An audio processing system according to claim 4 wherein the enhancement unit comprises an adaptive filter C(z,n) of the form

7. An audio processing system according to claim 1 comprising a probe signal generator for generating a probe signal contributing to the estimation of the feedback transfer function.

8. An audio processing system according to claim 7 wherein the probe signal generator is adapted to provide that the probe signal has predefined characteristics, and wherein the enhancement unit is adapted to provide a noise signal estimate output based on said characteristics.

9. An audio processing system according to claim 7 wherein the probe signal generator is adapted to provide that the probe signal has a correlation time N₀ which is smaller than or equal to the sum of the forward path and feedback path delays, e.g. ≦

5 ms, such as ≦

64 samples.

10. An audio processing system according to claim 7 wherein the algorithm part of the feedback path estimation unit comprises a step length control block for controlling the step length of the algorithm in a given frequency region, and wherein the step length control block receives a control input from the probe signal generator.

11. An audio processing system according to claim 7 wherein the probe signal generator is adapted to provide a probe signal based on masked added noise.

12. An audio processing system according to claim 11 wherein the probe signal generator comprises an adaptive filter for filtering a white noise input sequence w, the output of the variable part M of the adaptive filter forming the masked probe signal, and the variable part M of the adaptive filter being updated based on a signal from the forward path by an algorithm part comprising a model of the human auditory system.

13. An audio processing system according to claim 7 wherein the probe signal generator is adapted to provide a probe signal based on perceptual noise substitution, PNS.

14. An audio processing system according to claim 7 wherein the enhancement unit is adapted to base the noise signal estimate output on an adaptive filter, e.g. a long-term prediction, LTP, filter D(z,n) adapted for filtering a feedback corrected input signal on the input side of the forward path to provide a noise signal estimate output comprising noise-like signal components said feedback corrected input signal.

15. An audio processing system according to claim 14 wherein the adaptive filter is a linear, finite impulse response (FIR) type filter with a time varying long-term prediction, LTP, filter characteristic of the specific form

16. An audio processing system according to claim 7 wherein the enhancement unit is adapted to provide a noise signal estimate output based on binaural prediction filtering, wherein an adaptive noise retrieval filter E is adapted for filtering a signal y_c from another microphone, e.g. from the input side of the forward path of a contra-lateral listening device.

17. An audio processing system according to claim 16 wherein the adaptive noise retrieval filter E has a time varying filter characteristic described by the difference equation

18. An audio processing system according to claim 7 comprising a master enhancement unit on the input side and a slave enhancement unit on the output side each enhancement unit being electrically connected to the feedback estimation unit, wherein the slave enhancement unit is adapted to provide the same transfer function as the master enhancement unit.

22. Use of an audio processing system according to claim 1 in a communication device or in a listening device or in an audio delivery system or in connection with active noise control.

23. Use according to claim 22 in connection with a low delay acoustic system wherein a delay between input and output transducer is less than 50 ms, such as less than 20 ms, such as less than 10 ms, such as less than 5 ms, such as such than 2 ms.

19. A method of estimating a feedback transfer function in an audio processing system comprising a feedback estimation system for estimating acoustic feedback, the hearing device comprising

a forward path between an input transducer and an output transducer and comprising a signal processing unit adapted for processing an SPU-input signal originating from the electric input signal and to provide a processed SPU-output signal u,

an electric feedback loop from the output side to the input side comprising a feedback path estimation unit for estimating the feedback transfer function from the output transducer to the input transducer,

the method comprising

extracting characteristics of the electric signal of the forward path and providing an estimated characteristics output;

adapting the feedback path estimation unit to use the estimated characteristics output in the estimation of the feedback transfer function.

20. A tangible computer-readable medium storing a computer program comprising program code means for causing a data processing system to perform the steps of the method according to claim 19, when said computer program is executed on the data processing system.

21. A data processing system comprising a processor and program code means for causing the processor to perform the steps of the method according to claim 19.